water

Article

Physico-chemical Characteristics of Corrosion Scalesfrom Different Pipes in Drinking WaterDistribution Systems

Manjie Li 1, Zhaowei Liu 1,* and Yongcan Chen 1,2

1 State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084, China;limanjie14@mails.tsinghua.edu.cn (M.L.); chenyc@tsinghua.edu.cn (Y.C.)

2 Southwest University of Science and Technology, 59 Qinglong Road, Mianyang 621010, Sichuan, China* Correspondence: liuzhw@tsinghua.edu.cn; Tel.: +86-133-6678-6870

Received: 20 June 2018; Accepted: 12 July 2018; Published: 13 July 2018�����������������

Abstract: Corrosion scales formed on iron pipe surfaces are an important factor defining waterquality in drinking water distribution systems, since they would release contaminants and causewater discoloration at transient hydrodynamic regimes. Consequently, characterization of corrosionscales is indispensable to water quality protection. In this study, corrosion products were carefullycollected from three old, corroded iron pipes made of different materials and exposed to differentwater qualities and operation conditions. Physico-chemical characteristics of these scales weredetermined using Scanning Electron Microscope (SEM), Energy Dispersive X-ray Spectroscopy (EDS),Inductively Coupled Plasma (ICP), X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy(XPS). Testing results show that scale characteristics, including micromorphology, porosity andcomposition, vary significantly due to different pipe materials, water qualities and hydraulicconditions. Zinc coatings in galvanized pipes contribute to metal corrosion prevention, while attentionshould be paid to zinc release. High corrosive surface water facilitates the formation of developedcorrosion tubercles, in which the compact shell-like layer conduces to maintain the structural stabilityof corrosion scales under disturbance. Structural breaks and low-velocity zones in water distributionsystems might be in high potential of contaminant release, since the inhomogeneous materials andunusual hydraulic conditions would result in unstable scale characteristics.

Keywords: scale characteristics; water qualities; hydraulic conditions; scale structures; pipe materials;water distribution systems

1. Introduction

Corrosion scales affect water quality in drinking water distribution systems via severalmechanisms. They act as sinks for contaminant accumulation, harbor microbial growth and as aresult they may release contaminants back to the ambient water causing the deterioration of its quality,especially in changing hydrodynamic conditions [1–15]. Accordingly, characterization of corrosionscales is essential to understand metal release processes in drinking water networks and to protectwater quality.

Metal corrosion in pipe distribution systems includes general corrosion-producing uniform scalesand local corrosion which results in the development of corrosion tubercles [16–20]. Typical maturecorrosion tubercles usually consist of four layers: a corroded metal floor, an inner porous core layerdirectly contacting with pipe wall, a compact shell-like layer enveloping the core layer and a looselydeposited layer on solid-liquid interface [4,19–21]. Scales sampled from different layers of the sametubercle have different microscopic features and compositions. Generally, the inner core layer contains

Water 2018, 10, 931; doi:10.3390/w10070931 www.mdpi.com/journal/water

Water 2018, 10, 931 2 of 14

ferrous and ferric materials, while the outer shell-like layer, which contacts with bulk water that isabundant in dissolved oxygen, mainly consists of ferric ones [4,20].

Physical and chemical characteristics of corrosion scales remarkably influence the variation ofwater quality during water delivery [3,4,8,13,14,21,22]. Fe, O and C are confirmed as the predominantelements in corrosion scales formed on old, corroded iron pipes, with notable levels of Ca, S, Mn, Zn, P,Mg, Al and trace metal elements [1,10,14,19,22,23]. Efforts to identify crystalline phases comprisingsuch scales have shown that goethite (α-FeOOH), magnetite (Fe3O4) and lepidocrocite (γ-FeOOH)tend to be dominant, while siderite (FeCO3), hematite (Fe2O3), green rusts (hydrated ferrous-ferriccompounds containing CO3

2−, Cl− or SO42−), calcite (CaCO3) and quartz (SiO2) are also frequently

found [1–3,7,10,14,19–28].Extensive research efforts have been given to the characterization of corrosion scales and

hav efound that scale properties vary significantly depending on pipe materials, water qualitiesand hydraulic conditions [1,3,4,10,11,13,14,19,21–23,29–31]. However, when investigating the scaleformation process and contaminant accumulation-release mechanisms, factors that influence theseprocesses are not supposed to be discussed separately. Corrosion scales were sampled from practicalpipe distribution systems, in which the environmental factors are complicated and interactive.

Generally, the corrosion scale layer is much thinner in galvanized iron pipes than in unlinedcast iron pipes [22]. Significant amounts of Zn and zincite (ZnO) have been frequently detected forscales formed on galvanized iron [14,19,22,23]. In addition to the accumulation from water, releaseand re-deposition of heavy metals, such as Cu and Mn, from pipe fittings or system joints also lead tothe deposition of tenorite (CuO) and manganese oxide (MnO) in some scales [11,14,21].

Ca and calcite have been consistently observed in corrosion scales formed on pipes exposedto water with high hardness [10,14,32,33]. Surface water, which usually contains more chloride andsulfate and is identified as a corrosive water source, conduces to form compact corrosion scales withhigh magnetite content [21,29]. It is reported that corrosion scales in pipes transporting groundwaterare relatively thin and smooth as well as have a higher content of amorphous iron materials [21].

Increased shear stress applied to the corrosion scales by increased flow results in notable impactson scale characteristics and its water discoloration potential [31,34–39]. Furthermore, hydraulicconditions influence the convective transport to the surface, diffusion of oxidants at the interface,as well as the transfer of species accelerating or inhibiting corrosion of pipe surfaces [3,4,14,29,31,40].All these processes are critical to determining the rate of growth and characteristics of thecorrosion scales.

In this study, three pipe systems typical for different pipe materials, pipe operation conditionsand transporting water qualities were adopted for comparison. Corrosion scales were sampled fromthe old, corroded iron pipes and characterized using several sophisticated techniques. The primaryobjective is to investigate how the combination of internal-external factors influences metal corrosionand scale development in pipes, which is essential to protect water quality stability in drinking waterdistribution systems.

2. Materials and Methods

2.1. Sample Collection

Nine groups of corrosion scale samples were collected from different pipes, different positions indistribution systems and different layers of scales, as summarized in Table 1. Figure 1 shows the pipesections and the corrosion scales on pipe surfaces.

Samples 1#, 2# and 3# were taken from a hybrid pipe section (Pipe A), which was described indetail in the previous research [14]. This pipe, which is assembled of an unlined cast iron sectionand galvanized iron section by a welded joint, served an experimental water delivery system inTsinghua University, Beijing, China and transported water intermittently as required. Pipe A isan excellent research object highly suitable for comparing corrosion mechanisms of different pipe

Water 2018, 10, 931 3 of 14

materials exposed to the same water quality [14] and ascertaining the element release from pipe jointsand their re-deposition on the adjacent surfaces. Samples 1# and 3# were taken from unlined cast ironpipe and galvanized iron pipe segments, respectively. Sample 2# was from the hybrid pipe section,which contained the welded joint (Figure 1b).

Samples 4# and 5# were derived from different positions of a galvanized iron service pipe system(Pipe B) in Tsinghua University. Sample 4# was taken from the screw connecting section (Figure 1c),which is out of zinc coating locally, while Sample 5# was from the straight pipe section (Figure 1d).

Pipe C originates from the water delivery trunk mains, which were made of unlined cast iron,in Zhengzhou City, Henan Province, China. Samples 6#, 7# and 8# were representative for thecorrosion tubercles, as shown in Figure 1e, while Sample 9# was prepared from the flaky scales.Samples 6#, 7# and 8# were taken from the inner, middle and outer layers of well-developed corrosiontubercles, respectively.

Water 2018, 10, x FOR PEER REVIEW 3 of 14

galvanized iron pipe segments, respectively. Sample 2# was from the hybrid pipe section, which contained the welded joint (Figure 1b).

Samples 4# and 5# were derived from different positions of a galvanized iron service pipe system (Pipe B) in Tsinghua University. Sample 4# was taken from the screw connecting section (Figure 1c), which is out of zinc coating locally, while Sample 5# was from the straight pipe section (Figure 1d).

Pipe C originates from the water delivery trunk mains, which were made of unlined cast iron, in Zhengzhou City, Henan Province, China. Samples 6#, 7# and 8# were representative for the corrosion tubercles, as shown in Figure 1e, while Sample 9# was prepared from the flaky scales. Samples 6#, 7# and 8# were taken from the inner, middle and outer layers of well-developed corrosion tubercles, respectively.

Figure 1. Pipe sections and corrosion scale samples: (a) unlined cast iron pipe section of Pipe A; (b) hybrid pipe section of Pipe A; (c) screw section of Pipe B; (d) straight section of Pipe B and (e) Pipe C. Figure 1. Pipe sections and corrosion scale samples: (a) unlined cast iron pipe section of Pipe A;(b) hybrid pipe section of Pipe A; (c) screw section of Pipe B; (d) straight section of Pipe B and(e) Pipe C.

Water 2018, 10, 931 4 of 14

Table 1. Corrosion scale samples.

Pipe ID Sample ID Pipe Material Pipe Diameter (cm) Pipe Age (Years) Water Source Service Position Sampling Area

A

1# Unlined cast iron

8.0 20 GroundwaterExperimental water

delivery system /2# Hybrid pipe

3# Galvanized iron

B4#

Galvanized iron 3.5 20 Groundwater Service pipes Screw section

5# Straight section

C

6#

Unlined cast iron 100 30 Surface water Trunk mains

Inner layer of corrosion tubercle

7# Middle layer of corrosion tubercle

8# Outer layer of corrosion tubercle

9# Flaky scale

Water 2018, 10, 931 5 of 14

2.2. Water Quality

The water sources supplied in Tsinghua University and Zhengzhou City are groundwater andsurface water, respectively. In the past, water supplied in Zhengzhou City was taken from the YellowRiver. Since December, 2014, the water source has been switched into the Danjiangkou Reservoir watertransferred by the South-to-North Water Diversion Project. Corrosion scales in Pipe C were sampledand analyzed on April, 2016. In other words, Pipe C has worked with the Yellow River water for nearly30 years and the Danjiangkou Reservoir water for around 1.5 years before replacement.

Summarization of source water quality is given in Table 2. The primary difference is the abundanceof chloride and sulfate in the Yellow River water, which could be considered as high corrosivity wateraccording to its Larson Ratio [29]. It is also reported that exposure of iron to highly corrosive waterleads to the formation of developed corrosion scales [21]. As shown in Figure 1, Pipe C was moreheavily corroded than Pipe A and Pipe B. Corrosion scales in Pipe C were more developed, with theexistence of layered tubercles.

Table 2. Source water quality.

Water Sources pH Hardness(mg/L as CaCO3)

Turbidity(NTU)

SO42−

(mg/L)Cl−

(mg/L)Al

(mg/L)

Tsinghua University Groundwater 7.93 196.2 0.32 50.0 18.6 <0.02

Zhengzhou City Yellow River 7.98 235.8 0.19 85.6 54.2 0.07

Danjiangkou Reservoir 8.05 155.5 0.22 41.1 18.8 0.07

2.3. Scale Analysis

Corrosion scale samples were dried under vacuum at about 20 ◦C immediately after extraction.Thereafter, they were pulverized using an agate mortar and pestle for analysis.

Microstructures and elemental compositions of the scale samples were observed by ScanningElectron Microscope and Energy Dispersive X-ray Spectroscopy (SEM-EDS, Hitachi SU8010, Tokyo,Japan). SEM technique is conducted under high vacuum condition and used for material analyses ofsurface micromorphology and particle size. Elemental composition in a small area can be qualitativelyor quantitively measured by EDS to determine the solid phase.

The NOVA 2000e surface area and porosity analyzer (Quantachrome Co., Boynton Beach, FL,USA) was used to determine the specific surface area and porosity of the corrosion scales by N2 gasadsorption and desorption. The scale samples were degassed at 120 ◦C prior to analysis.

The method of Inductively Coupled Plasma (ICP) is widely used in analytical chemistry toquantitively determine the inorganic elements in solutions and digested solids. Scale samples weredigested by inorganic acid and analyzed by ICP (Agilent 7500ce, Santa Clara, CA, USA) to investigatethe elemental composition.

The X-ray Diffraction (XRD, Bruker D8 Advance, Bruker, Germany) was conducted on the powdersamples using Cu Kα radiation, with the 2θ ranged from 10◦ to 80◦. Diffraction data determined byXRD were compared against reference patterns from International Center for Diffraction Data (ICDD)to identify the crystalline phase composition of corrosion scales.

X-ray Photoelectron Spectroscopy (XPS) technique, with the sampling depth of only severalnanometers, is surface-sensitive and always used for qualitive determination and valence analysisof surface elements in solid samples. Corrosion scales were determined by XPS (Thermo ScientificK-Alpha, Fremont, CA, USA) analysis and comparison with the NIST XPS Database. Non-linear leastsquares fitting of deconvolution of Fe (2p3/2) was conducted to investigate the relative amount ofdifferent iron species in the samples.

Water 2018, 10, 931 6 of 14

3. Results

3.1. Physical Characteristics

3.1.1. Apparent Description

As shown in Figure 1a,b, corrosion scale layers inside Pipe A are very thin, with a thickness ofonly a few millimeters or less than a millimeter. These dark black samples are hard and difficult to bepulverized. Sample 2# seems more developed, compared with Sample 1# and Sample 3#, and thereappears to be some bumps.

On the contrary, scale samples from Pipe B, presenting yellowish-brown, are easy to be peeledoff and pulverized. It can be found from Figure 1c,d that scale layer is much thicker inside the screwsection than inside the straight section. No layered structure could be identified in the corrosion scalesfrom Pipe A or Pipe B.

As shown in Figure 1e, corrosion scale layers in Pipe C are relatively well developed. All over thepipe surfaces were covered with flaky scales resulting from uniform corrosion. These flaky scales arecrisp and easy to be peeled off. They are dark gray and some have an inapparent yellowish interlayer.Tuberculate scales due to local corrosion dispersedly distribute in the pipe surfaces, with an averagethickness of 3 centimeters and an obvious layered structure.

Taken from the inner layer of the corrosion tubercle, Sample 6# is flake-like and fragile. Adjacentto Sample 6#, the core layer Sample 7# is enveloped by a shell layer. The inner flake layer and middlecore layer are both yellowish-brown. The shell-like layer is hard and dark black, surrounding the corelayer. There are some yellowish-brown particulate deposits loosely attached to the outside surface ofthe shell-like layer. Due to the difficulty in separating out the shell-like layer, Sample 8# is a mixtureof the dominant shell-like layer and a small quantity of the inner core layer and outer deposit layer.Sample 9# was prepared from the flaky scales.

3.1.2. Micromorphology

SEM examination results show that distinctly diverse microstructures can be found in the scalesfrom Pipe A, as discussed in detail by Li et al. [14]. Sample 1# presents lamellar, plate-like andneedle-like. The micromorphology of A-HP samples shows the presence of highly diverse lamellar,needle-like, crystal-like, columnar, cottony and blocky formations. Crystal-like, lamellar, cottony,filamentous and porous spongy structures were observed in Sample 3#.

Figure 2 shows SEM micrographs of the corrosion scale samples from Pipe B and Pipe C.In contrast, the micromorphology of Pipe B and Pipe C samples seems nondescript. Some needle-likeand flaky structures were observed for Sample 4# and Sample 5#. Sample 6# and Sample 8# appearglobular, while Sample 7# and Sample 9# present cottony.

Water 2018, 10, x FOR PEER REVIEW 6 of 14

3. Results

3.1. Physical Characteristics

3.1.1. Apparent Description

As shown in Figure 1a,b, corrosion scale layers inside Pipe A are very thin, with a thickness of only a few millimeters or less than a millimeter. These dark black samples are hard and difficult to be pulverized. Sample 2# seems more developed, compared with Sample 1# and Sample 3#, and there appears to be some bumps.

On the contrary, scale samples from Pipe B, presenting yellowish-brown, are easy to be peeled off and pulverized. It can be found from Figure 1c,d that scale layer is much thicker inside the screw section than inside the straight section. No layered structure could be identified in the corrosion scales from Pipe A or Pipe B.

As shown in Figure 1e, corrosion scale layers in Pipe C are relatively well developed. All over the pipe surfaces were covered with flaky scales resulting from uniform corrosion. These flaky scales are crisp and easy to be peeled off. They are dark gray and some have an inapparent yellowish interlayer. Tuberculate scales due to local corrosion dispersedly distribute in the pipe surfaces, with an average thickness of 3 centimeters and an obvious layered structure.

Taken from the inner layer of the corrosion tubercle, Sample 6# is flake-like and fragile. Adjacent to Sample 6#, the core layer Sample 7# is enveloped by a shell layer. The inner flake layer and middle core layer are both yellowish-brown. The shell-like layer is hard and dark black, surrounding the core layer. There are some yellowish-brown particulate deposits loosely attached to the outside surface of the shell-like layer. Due to the difficulty in separating out the shell-like layer, Sample 8# is a mixture of the dominant shell-like layer and a small quantity of the inner core layer and outer deposit layer. Sample 9# was prepared from the flaky scales.

3.1.2. Micromorphology

SEM examination results show that distinctly diverse microstructures can be found in the scales from Pipe A, as discussed in detail by Li et al. [14]. Sample 1# presents lamellar, plate-like and needle-like. The micromorphology of A-HP samples shows the presence of highly diverse lamellar, needle-like, crystal-like, columnar, cottony and blocky formations. Crystal-like, lamellar, cottony, filamentous and porous spongy structures were observed in Sample 3#.

Figure 2 shows SEM micrographs of the corrosion scale samples from Pipe B and Pipe C. In contrast, the micromorphology of Pipe B and Pipe C samples seems nondescript. Some needle-like and flaky structures were observed for Sample 4# and Sample 5#. Sample 6# and Sample 8# appear globular, while Sample 7# and Sample 9# present cottony.

Figure 2. SEM micrographs of corrosion scales from Pipe B and Pipe C. (a–f) represent Sample 4#–9#, respectively. Figure 2. SEM micrographs of corrosion scales from Pipe B and Pipe C. (a–f) represent

Sample 4#–9#, respectively.

Water 2018, 10, 931 7 of 14

3.1.3. Surface Area and Porosity

Physical characteristics of the scale samples were further analyzed based on the nitrogenadsorption and desorption studies. Specific surface area, pore volume and pore size were calculatedby several methods and the results were shown in Table 3. Due to the limited amount of scale samplesfrom Pipe A and the difficulty in peeling off, surface area and porosity analysis results of these samplesare absent.

On the whole, specific area, pore volume and pore size of the scales from Pipe B are all higher thanthose from Pipe C, indicating the comparative porosity of Sample 4# and Sample 5#. Sample 4# has arelatively lower BET-specific surface area and a higher average BET pore diameter than Sample 5#does, which manifests in that the pore size of Sample 4# is a bit higher.

As for Pipe C, the orders of surface area and pore volume appear Sample 9# < Sample 6# <Sample 8# < Sample 7#, while that of pore size shows Sample 8# ≈ Sample 7# < Sample 6# < Sample 9#.This indicates that the pores of Sample 6# and Sample 9# are larger in size but fewer in amount,resulting in the lower specific surface area.

Table 3. Surface area and porosity of scale samples.

Surface Area (m2/g) Pore Volume (×103 cm3/g) Pore Size (Å)

BET a BJH b BJH b BET a BJH b

Pipe B 4# 123.0 100.5 219.0 58.6 87.25# 170.1 89.4 174.1 40.0 77.9

Pipe C

6# 50.5 59.7 87.8 54.0 58.87# 81.7 87.3 104.6 41.4 47.68# 77.4 87.4 98.4 41.3 45.09# 31.8 36.7 58.0 73.3 62.6

Note: a Calculated by using the Brunauer, Emmett and Teller (BET) method. b Calculated based on desorption databetween 10-3000 Å diameter by using the Barrett, Joyner and Halenda (BJH) method.

3.2. Chemical Characteristics

3.2.1. Elemental Composition

The elemental compositions of scale samples determined by ICP are shown in Table 4. For eachgroup of scales, more than three samples were measured and the average was given. Figure 3 illustratesthe cumulative occurrence profiles of Fe, Ca and Zn determined by EDS. In consideration of the samplequantities for cumulative analysis, six profiles were plotted for each element, representing Sample 1#,Sample 2#, Sample 3#, scales from Pipe B, scale from Pipe C and the total deposits, respectively.

In general, Fe, O and C are the most prevalent elemental components in the corrosion scales,followed by Ca, Zn, S, Al, Si and so on. However, there is significant diversity among the scales fromdifferent pipes. Even apparent differences can be identified in the corrosion scales from different areasof the same pipe.

In Pipe A, elemental composition of Sample 1# is relatively uniform, while there is an evidentdiversity among Sample 2#, as shown in Figure 3. Fe content in Sample 1# is a bit higher than those inSample 2# and Sample 3#. Fe content in Sample 2# covers a wide range (Figure 3a). Ca is prominentin the scales from Pipe A, especially in Sample 2# (Table 4). Cumulative Ca distribution profile inSample 2# covers an extremely wide range (Figure 3b). Zn mainly exists in the scales from galvanizediron pipes, namely Sample 3#, Sample 4# and Sample 5# (Table 4). Sample 3# exhibits the dominanceof Zn content (Figure 3c). Corrosion scales from Pipe A are comparatively abundant in Si, Mn, Pb, Tiand Cu, especially for Sample 2# (Table 4).

From Figure 3b,c, similar trends in distributions of Ca and Zn can be observed in the same groupof samples, especially in Sample 1# and Sample 3#. However, the trends are significantly different

Water 2018, 10, 931 8 of 14

between Sample 1# and Sample 3#. As shown in Figure 3c, there are two plateaus (40~50 mg/g and420~470 mg/g) in the Zn distribution profile of Sample 3#. As for the Ca distribution profile of Sample1# shown in Figure 3b, only one plateau (10~15 mg/g) is observed.

In Pipe C, one of the distinctive characteristics is that the order of S content shows Sample 6# >Sample 9# > Sample 7# > Sample 8#, as shown in Table 4. S content detected in Sample 6# reachesseveral times or even several tens of times of those in other scale samples. Compared with those fromPipe A and Pipe B, the scales extracted from Pipe C are relatively rich in S, Al, K and P.

Table 4. Elemental composition of scale samples by ICP test.

Elemental Composition(mg/g) Fe Ca Zn S Al Si Mg Mn Pb K P Na Ti Cu

Pipe A 1#2#3#

554.3283.4386.5

41.7185.823.4

0.83.1

158.9

0.51.01.8

0.20.80.9

3.45.84.8

1.03.41.2

2.73.11.5

0.68.52.5

0.10.70.2

///

/0.20.1

/2.2/

1.00.50.4

Pipe B 4#5#

648.9612.8

1.00.6

31.314.6

4.64.3

0.10.1

1.71.3

0.4/

0.30.2

0.30.2

0.10.1

0.80.3

0.20.2

//

//

Pipe C 6#7#8#9#

337.4211.4316.8254.2

5.72.43.28.7

0.10.10.1/

68.711.25.7

15.7

3.96.46.32.8

1.21.60.81.6

1.92.52.31.7

2.10.20.61.5

////

1.12.02.00.8

0.81.81.50.8

0.60.70.90.2

0.20.40.30.1

////

Note: The symbol/represents the corresponding element was detected below 0.1 mg/g.

Water 2018, 10, x FOR PEER REVIEW 8 of 14

420~470 mg/g) in the Zn distribution profile of Sample 3#. As for the Ca distribution profile of Sample 1# shown in Figure 3b, only one plateau (10~15 mg/g) is observed.

In Pipe C, one of the distinctive characteristics is that the order of S content shows Sample 6# > Sample 9# > Sample 7# > Sample 8#, as shown in Table 4. S content detected in Sample 6# reaches several times or even several tens of times of those in other scale samples. Compared with those from Pipe A and Pipe B, the scales extracted from Pipe C are relatively rich in S, Al, K and P.

Table 4. Elemental composition of scale samples by ICP test.

Elemental Composition (mg/g)

Fe Ca Zn S Al Si Mg Mn Pb K P Na Ti Cu

Pipe A 1# 2# 3#

554.3 283.4 386.5

41.7 185.8 23.4

0.8 3.1

158.9

0.5 1.0 1.8

0.2 0.8 0.9

3.4 5.8 4.8

1.0 3.4 1.2

2.7 3.1 1.5

0.6 8.5 2.5

0.1 0.7 0.2

/ / /

/ 0.2 0.1

/ 2.2 /

1.0 0.5 0.4

Pipe B 4# 5#

648.9 612.8

1.0 0.6

31.3 14.6

4.6 4.3

0.1 0.1

1.7 1.3

0.4 /

0.3 0.2

0.3 0.2

0.1 0.1

0.8 0.3

0.2 0.2

/ /

/ /

Pipe C

6# 7# 8# 9#

337.4 211.4 316.8 254.2

5.7 2.4 3.2 8.7

0.1 0.1 0.1 /

68.7 11.2 5.7

15.7

3.9 6.4 6.3 2.8

1.2 1.6 0.8 1.6

1.9 2.5 2.3 1.7

2.1 0.2 0.6 1.5

/ / / /

1.1 2.0 2.0 0.8

0.8 1.8 1.5 0.8

0.6 0.7 0.9 0.2

0.2 0.4 0.3 0.1

/ / / /

Note: The symbol / represents the corresponding element was detected below 0.1 mg/g.

Figure 3. Cont.

Water 2018, 10, 931 9 of 14Water 2018, 10, x FOR PEER REVIEW 9 of 14

Figure 3. Cumulative occurrence profiles of (a) Fe, (b) Ca and (c) Zn in corrosion scale samples.

3.2.2. Crystalline Compounds

XRD patterns of the corrosion scales are shown in Figure 4. It can be found that the crystalline compounds of scale samples are quite different from pipe to pipe. Iron materials in these corrosion scales mainly exist in amorphous state.

In Pipe A, calcite (CaCO3) is the predominant crystal in all samples, followed by magnetite (Fe3O4), maghemite (γ-Fe2O3) and goethite (α-FeOOH). Intensity of calcite in Sample 2# is much higher than those in Sample 1# and Sample 3#, while intensities of magnetite and maghemite are relatively higher in Sample 1#. Tenorite (CuO) and manganese oxide (MnO) were detected in Sample 2#, while zincite (ZnO) and hydrozincite (Zn5(CO3)2(OH)6) were detected in Sample 3#.

Crystalline compounds in the scales from Pipe B are simpler and more uniform. Goethite is the primary crystal in Sample 4# and Sample 5#, with the existence of zincite and hydrozincite.

As for Pipe C, quartz (SiO2) is the dominant crystalline compound in all scales, though distinct differences could be found among different samples. Intensity of quartz in Sample 6# is the lowest, while the highest one was detected in Sample 9#. Lepidocrocite (γ-FeOOH) was observed in Sample 6#; goethite was found in Sample 7#; magnetite, maghemite and goethite were detected in Sample 8#.

Figure 4. XRD patterns of corrosion scales. (a–i) represent Sample 1#–9#, respectively.

Figure 3. Cumulative occurrence profiles of (a) Fe, (b) Ca and (c) Zn in corrosion scale samples.

3.2.2. Crystalline Compounds

XRD patterns of the corrosion scales are shown in Figure 4. It can be found that the crystallinecompounds of scale samples are quite different from pipe to pipe. Iron materials in these corrosionscales mainly exist in amorphous state.

In Pipe A, calcite (CaCO3) is the predominant crystal in all samples, followed by magnetite (Fe3O4),maghemite (γ-Fe2O3) and goethite (α-FeOOH). Intensity of calcite in Sample 2# is much higher thanthose in Sample 1# and Sample 3#, while intensities of magnetite and maghemite are relatively higherin Sample 1#. Tenorite (CuO) and manganese oxide (MnO) were detected in Sample 2#, while zincite(ZnO) and hydrozincite (Zn5(CO3)2(OH)6) were detected in Sample 3#.

Crystalline compounds in the scales from Pipe B are simpler and more uniform. Goethite is theprimary crystal in Sample 4# and Sample 5#, with the existence of zincite and hydrozincite.

Water 2018, 10, x FOR PEER REVIEW 9 of 14

Figure 3. Cumulative occurrence profiles of (a) Fe, (b) Ca and (c) Zn in corrosion scale samples.

3.2.2. Crystalline Compounds

XRD patterns of the corrosion scales are shown in Figure 4. It can be found that the crystalline compounds of scale samples are quite different from pipe to pipe. Iron materials in these corrosion scales mainly exist in amorphous state.

In Pipe A, calcite (CaCO3) is the predominant crystal in all samples, followed by magnetite (Fe3O4), maghemite (γ-Fe2O3) and goethite (α-FeOOH). Intensity of calcite in Sample 2# is much higher than those in Sample 1# and Sample 3#, while intensities of magnetite and maghemite are relatively higher in Sample 1#. Tenorite (CuO) and manganese oxide (MnO) were detected in Sample 2#, while zincite (ZnO) and hydrozincite (Zn5(CO3)2(OH)6) were detected in Sample 3#.

Crystalline compounds in the scales from Pipe B are simpler and more uniform. Goethite is the primary crystal in Sample 4# and Sample 5#, with the existence of zincite and hydrozincite.

As for Pipe C, quartz (SiO2) is the dominant crystalline compound in all scales, though distinct differences could be found among different samples. Intensity of quartz in Sample 6# is the lowest, while the highest one was detected in Sample 9#. Lepidocrocite (γ-FeOOH) was observed in Sample 6#; goethite was found in Sample 7#; magnetite, maghemite and goethite were detected in Sample 8#.

Figure 4. XRD patterns of corrosion scales. (a–i) represent Sample 1#–9#, respectively. Figure 4. XRD patterns of corrosion scales. (a–i) represent Sample 1#–9#, respectively.

Water 2018, 10, 931 10 of 14

As for Pipe C, quartz (SiO2) is the dominant crystalline compound in all scales, though distinctdifferences could be found among different samples. Intensity of quartz in Sample 6# is the lowest,while the highest one was detected in Sample 9#. Lepidocrocite (γ-FeOOH) was observed in Sample 6#;goethite was found in Sample 7#; magnetite, maghemite and goethite were detected in Sample 8#.

Non-linear regression using a Gaussian/Lorentzian curve was conducted on deconvolution ofthe Fe (2p3/2) peak determined by XPS. The peak area stands for the relative amount of iron species incorrosion scales [22]. Table 5 shows the relative percentage of magnetite, maghemite and goethite onthe surfaces of scale samples.

The proportion of iron species is comparatively uniform in the corrosion scales from the samepipes. In Pipe A, goethite is prominent in Sample 1#, while magnetite is significant in Sample 2#. In thecorrosion scales from Pipe B, goethite is the primary iron compound, corresponding with the XRDresults. The content of magnetite and maghemite is significantly higher in Sample 4# than in Sample 5#.This corresponds to the findings by Yang et al. [21] that thicker corrosion scale has a higher ratio ofmagnetite/goethite. The relative proportions of magnetite, maghemite and goethite are approximatein the scales from Pipe C.

Table 5. Non-linear least squares fitting results of deconvolution of Fe(2p3/2) peak determined by XPS.

Chemical Composition (%) Magnetite (Fe3O4) Maghemite (γ-Fe2O3) Goethite (α-FeOOH)

Pipe A1#2#3#

174632

292226

543242

Pipe B 4#5#

90

3524

5676

Pipe C

6#7#8#9#

35282720

26292738

39434642

4. Discussion

Physico-chemical characteristics of corrosion scales vary significantly due to pipe materials, sourcewater qualities, hydraulic conditions, as well as scale structures. These internal and external factorsare interactive and exert a comprehensive effect on the processes of scale formation and development.

4.1. Pipe Materials

While investigating the corrosion mechanisms of different pipe materials, scale samples fromPipe A are the ideal research objects, since the same historical operation conditions of hydraulic, waterquality and climatic environment could be strictly confirmed. Comparison of the physico-chemicalcharacteristics of Sample 1#, Sample 2# and Sample 3# determined by SEM-EDS, ICP and XRD waselaborated in a previous research [14].

In general, micromorphology and chemical composition of the scales from unlined cast iron pipesare relatively uniform. Higher Fe content and intensities of magnetite and maghemite were detectedin these samples. The diverse microstructures and inhomogeneous components in the scales fromhybrid pipes can be interpreted as the combined structural and material effects of the welded joint.Therefore, more attention should be paid to the connecting areas in water distribution systems, whichmight release contaminants and give rise to colored water under disturbance.

In galvanized iron pipes, zinc coatings contribute to the prevention of metal corrosion, drawn fromFigure 1c,d (Pipe screw connecting section is out of zinc coatings locally.). In the service pipes whosecross section is usually narrow, more attention should be paid to the iron corrosion and subsequent pipeobstruction as well as energy loss in some specific positions, such as screw junctions and elbows, whichare generally out of zinc coatings. However, zinc coatings would result in higher zinc compounds incorrosion scales and zinc release, which should also come into notice [14].

Water 2018, 10, 931 11 of 14

4.2. Water Sources

Corrosion scales in this study were collected from different pipe distribution systems, whosesource water qualities differ, as shown in Table 1. Surface water source from the Yellow River isabundant in chloride and sulfate and identified as high corrosive water.

Compared with those from the other two pipes, corrosion scale layers in Pipe C are far moredeveloped. According to Table 3, scale samples in Pipe B possess higher porosity than those in PipeC do, from the perspectives of specific area, pore volume and pore size. This verifies that the highcorrosive surface water leads to the formation of developed and compact corrosion scales [21].

The high abundance of Ca (Table 4 and Figure 3b) in scales from Pipe A is correlated with theprevalence of calcite (Figure 4a–c). This may be associated with the precipitation of calcium carbonatefrom the high-alkalinity groundwater source [10,14,32,33].

Comparatively, corrosion scales extracted from Pipe C are rich in S (Table 4). As shown in Table 1,water from the Yellow River, which Pipe C historically transported for a long period of time, is rich insulfate and this appears to have led to the abundance of S in the corrosion scales. However, S contentis extremely different from layer to layer in the corrosion tubercles, appearing inner layer > middlelayer > outer layer, which resulted from the water source switch in Zhengzhou City. Water from theDanjiangkou Reservoir is low in sulfate and altered the composition of corrosion scales, even thoughthe exposure period is less than 1.5 years. Owing to the protection from the compact shell-like layer,the effect on the inner layer is relatively insignificant, resulting in the stratification of S content incorrosion tubercles.

4.3. Structures of Corrosion Scales

In Pipe C, the flaky scale layers resulted from uniform corrosion and the tuberculate scales due tolocal corrosion possess totally different characteristics.

The flaky scales, namely Sample 9#, have the smallest specific surface area and pore volumeand the biggest pore size, indicating that the pores are larger in size but fewer in amount. Similarlywith Sample 6#, the inner layer of the corrosion tubercles. The outer and middle layers of thecorrosion tubercle are relatively compact, with well-developed microporosity. This is consistent withthe typical layered corrosion tubercle model developed in the previous researches [4,19,21], verifyingthe protective effect of the outer shell-like layer.

According to Figure 4, iron materials detected in different layers of the corrosion tubercles alsodiffer, with lepidocrocite in the inner layer; goethite in the middle layer; and magnetite, maghemite,goethite in the outer layer. This verifies the previous report that magnetite and goethite are the mainiron substances in the dense shell-like layer [19,24].

Overall speaking, high corrosive surface water facilitates the formation of developed corrosiontubercles, in which the compact shell-like layer contributes to maintain the structural stability ofcorrosion scales under disturbance.

4.4. Hydraulic Conditions

Distinctly different from the normal yellowish-brown ones, the dark black corrosion scales inPipe A are very thin and difficult to be peeled off and pulverized. Besides, microstructures of thesescales are extremely diverse and non-uniform. This might be related to the stagnation condition oreven out-of-water that the pipe often underwent, since this pipe transported water intermittently tothe experimental system as required. Microbial activities under low oxygen conditions during thestagnant period might contribute to the strange apparent characteristics and micromorphology in PipeA, which needs further investigation.

As discussed afore, corrosion scales from Pipe A, which transported groundwater, are abundant incalcite. The relatively higher calcite content in corrosion scales from the pipes transporting groundwaterhave also been shown in other research [21]. However, to our knowledge, no research has found that

Water 2018, 10, 931 12 of 14

calcite plays a predominant role in crystalline compositions. Few calcium or calcite was determined inscale samples from Pipe B that transported the same water as Pipe A. It is speculated that the intermittentoperation condition in Pipe A contributes to the absorption of calcium and crystallization of calcite.

Zn content is significantly higher in Sample 3# than in Sample 4# (Table 4 and Figure 3c), despitethe fact that they were both scrapings from galvanized iron pipe sections. The flowing water in Pipe Brenews water body to deliver sufficient oxidants to the pipe surfaces [3,40], enhancing the oxidationof pipe metal and development of corrosion scales, resulting in the relatively low Zn proportion.Inversely, due to the low velocity and frequent stagnation condition, corrosion scales in Pipe A areundeveloped and unstable. The Zn distribution profile of Sample 3# with two plateaus, as shown inFigure 3c, indicates that Zn presents in the scales in two different corrosion statuses. Employed as asacrificial protective layer, zinc coating is oxidized prior to the underlying iron substrate, leading to theplateau of high Zn content in corrosion scales. After depletion of zinc coating locally, the exposed ironsubstrate is oxidized and participates in the development of corrosion scales, resulting in the plateauof low Zn content. This can be confirmed from Figure 3a, that two plateaus can also be observed in theFe distribution profile of Sample 3#.

As shown in Table 1, Pipe B is a galvanized service pipe. Sample 4# was taken from the screwconnecting section, which is out of zinc coatings locally and has a sudden increase in cross section.However, the contents of Zn and its compounds are significant in Sample 4#, they even exceed thosein Sample 5# (Table 4 and Figure 4). This indicates that zinc from the coatings or corrosion scales inthe straight pipe section would be dissolved or scoured off by the water flow and precipitate in thescrew section. The vortex brought about by the sudden change of flow pathway facilitates particleprecipitation [31,34–39]. It also indirectly reveals the release-deposit process of metal inside the pipedistribution systems [1–6,14].

In consequence, pipe operation and hydraulic conditions remarkably influence the formationof corrosion scales. In the low-velocity zones or dead ends of drinking water distribution systems,characteristics of corrosion scales are diverse and unstable. Structural breaks in the pipe distributionsystems, such as connection joints and elbows, would act as harbors for contaminant accumulationand development of corrosion tubercles, due to the combined effect of inhomogeneous pipe materialsand unusual hydraulic conditions. Stricter protection is required in these fragile areas, since they arein high risk of contaminant release in changing hydrodynamic conditions and might become threats tohuman health [3,14].

5. Conclusions

Physico-chemical characteristics of corrosion scales sampled from drinking water pipe systemswere determined using several sophisticated techniques. It can be concluded that scale characteristics,including micromorphology, porosity and composition, vary significantly due to different pipematerials, water qualities and hydraulic conditions. Zinc coatings in galvanized iron pipes wouldlead to zinc release in drinking water, though they are conductive to prevention of metal corrosion.Corrosion tubercles developed under high corrosive surface water possess stronger resistance tocontaminant release under disturbance, thanks to the protection of the compact shell-like layer. Stricterprotection is required to the structural breaks and low-velocity zones in pipe systems, where theinhomogeneous pipe materials and unusual hydraulic conditions would result in unstable scalecharacteristics. These areas are always in high risk of contaminant release under disturbance andmight become threats to human health. The findings in this study could be applied in protection ofwater quality stability in drinking water distribution systems.

Author Contributions: Conceptualization, M.L., Z.L. and Y.C.; Data curation, M.L. and Z.L.; Formal analysis, M.L.;Funding acquisition, Z.L. and Y.C.; Investigation, M.L.; Methodology, M.L., Z.L. and Y.C.; Project administration,Z.L. and Y.C.; Resources, Z.L. and Y.C.; Supervision, Z.L. and Y.C.; Validation, M.L. and Z.L.; Visualization, M.L.and Z.L.; Writing—original draft, M.L. and Z.L.; Writing—review & editing, M.L., Z.L. and Y.C.

Funding: This research received no external funding.

Water 2018, 10, 931 13 of 14

Acknowledgments: This work was supported by National Science and Technology Support Project of China(No. 2016YFC0502204), the National Natural Science Foundation of China (No. 51579130 and No. 91647116),and Tsinghua University Initiative Scientific Research Program (No. 2014z09112).

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Tuovinen, O.H.; Button, K.S.; Vuorinen, A.; Carlson, L.; Mair, D.M.; Yut, L.A. Bacterial, chemical, andmineralogical characteristics of tubercles in distribution pipelines. J. Am. Water Works Assoc. 1980, 72,626–635. [CrossRef]

2. Lytle, D.A.; Sorg, T.J.; Frietch, C. Accumulation of arsenic in drinking water distribution systems.Environ. Sci. Technol. 2004, 38, 5365–5372. [CrossRef] [PubMed]

3. Sarin, P.; Snoeyink, V.L.; Bebee, J.; Jim, K.K.; Beckett, M.A.; Kriven, W.M.; Clement, J.A. Iron release fromcorroded iron pipes in drinking water distribution systems: Effect of dissolved oxygen. Water Res. 2004, 38,1259–1269. [CrossRef] [PubMed]

4. Sarin, P.; Snoeyink, V.L.; Lytle, D.A.; Kriven, W.M. Iron corrosion scales: Model for scale growth, iron release,and colored water formation. J. Environ. Eng. 2004, 130, 364–373. [CrossRef]

5. Imran, S.A.; Dietz, J.D.; Mutoti, G.; Taylor, J.S.; Randall, A.A.; Cooper, C.D. Red water release in drinkingwater distribution systems. J. Am. Water Works Assoc. 2005, 97, 93–100. [CrossRef]

6. Imran, S.A.; Dietz, J.D.; Mutoti, G.; Xiao, W.Z.; Taylor, J.S.; Desai, V. Optimizing source water blends forcorrosion and residual control in distribution systems. J. Am. Water Works Assoc. 2006, 98, 107–115. [CrossRef]

7. Gerke, T.L.; Maynard, J.B.; Schock, M.R.; Lytle, D.L. Physiochemical characterization of five iron tuberclesfrom a single drinking water distribution system: Possible new insights on their formation and growth.Corros. Sci. 2008, 50, 2030–2039. [CrossRef]

8. Schock, M.R.; Hyland, R.N.; Welch, M.M. Occurrence of contaminant accumulation in lead pipe scalesfrom domestic drinking water distribution systems. Environ. Sci. Technol. 2008, 42, 4285–4291. [CrossRef][PubMed]

9. Kim, E.J.; Herrera, J.E. Characteristics of lead corrosion scales formed during drinking water distributionand their potential influence on the release of lead and other contaminants. Environ. Sci. Technol. 2010, 44,6054–6061. [CrossRef] [PubMed]

10. Peng, C.Y.; Korshin, G.V.; Valentine, R.L.; Hill, A.S.; Friedman, M.J.; Reiber, S.H. Characterization of elementaland structural composition of corrosion scales and deposits formed in drinking water distribution systems.Water Res. 2010, 44, 4570–4580. [CrossRef] [PubMed]

11. Peng, C.Y.; Hill, A.S.; Friedman, M.J.; Valentine, R.L.; Larson, G.S.; Romero, A.M.Y.; Reiber, S.H.; Korshin, G.V.Occurrence of trace inorganic contaminants in drinking water distribution systems. J. Am. Water Works Assoc.2012, 104, 181–193. [CrossRef]

12. Gerke, T.L.; Little, B.J.; Luxton, T.P.; Scheckel, K.G.; Maynard, J.B. Strontium concentrations in corrosionproducts from residential drinking water distribution systems. Environ. Sci. Technol. 2013, 47, 5171–5177.[CrossRef] [PubMed]

13. Wang, H.; Masters, S.; Edwards, M.A.; Falkinham, J.O.; Pruden, A. Effect of disinfectant, water age, and pipematerials on bacterial and eukaryotic community structure in drinking water biofilm. Environ. Sci. Technol.2014, 48, 1426–1435. [CrossRef] [PubMed]

14. Li, M.J.; Liu, Z.W.; Chen, Y.C.; Hai, Y. Characteristics of iron corrosion scales and water quality variations indrinking water distribution systems of different pipe materials. Water Res. 2016, 106, 593–603. [CrossRef][PubMed]

15. Trueman, B.F.; Gagnon, G.A. Understanding the role of particulate iron in lead release to drinking water.Environ. Sci. Technol. 2016, 50, 9053–9060. [CrossRef] [PubMed]

16. Larson, T.E.; King, R.M. Corrosion by water at low flow velocity. J. Am. Water Works Assoc. 1954, 46, 1–9.[CrossRef]

17. Larson, T.E.; Skold, R.V. Corrosion and tuberculation of cast iron. J. Am. Water Works Assoc. 1957, 49,1294–1301.

18. Obrecht, M.F.; Pourbaix, M. Corrosion of metals in potable water systems. J. Am. Water Works Assoc. 1967, 59,977–992. [CrossRef]

Water 2018, 10, 931 14 of 14

19. Sarin, P.; Snoeyink, V.L.; Bebee, J.; Kriven, W.M.; Clement, J.A. Physico-chemical characteristics of corrosionscales in old iron pipes. Water Res. 2001, 35, 2961–2969. [CrossRef]

20. Jin, J.T.; Wu, G.X.; Guan, Y.T. Effect of bacterial communities on the formation of cast iron corrosion tuberclesin reclaimed water. Water Res. 2015, 71, 207–218. [CrossRef] [PubMed]

21. Yang, F.; Shi, B.Y.; Gu, J.N.; Wang, D.S.; Yang, M. Morphological and physicochemical characteristics ofiron corrosion scales formed under different water source histories in a drinking water distribution system.Water Res. 2012, 46, 5423–5433. [CrossRef] [PubMed]

22. Tang, Z.J.; Hong, S.; Xiao, W.Z.; Taylor, J. Characteristics of iron corrosion scales established under blendingof ground, surface, and saline waters and their impacts on iron release in the pipe distribution system.Corros. Sci. 2006, 48, 322–342. [CrossRef]

23. Lin, J.P.; Ellaway, M.; Adrien, R. Study of corrosion material accumulated on the inner wall of steel waterpipe. Corros. Sci. 2001, 43, 2065–2081. [CrossRef]

24. Sontheimer, H.; Kolle, W.; Snoeyink, V.L. The siderite model of the formation of corrosion-resistant scales.J. Am. Water Works Assoc. 1981, 73, 572–579. [CrossRef]

25. Olowe, A.A.; Genin, J.M.R.; Bauer, P. Hyperfine interactions and structures of ferrous hydroxide and greenrust II in sulfated aqueous media. Hyperfine Interact. 1988, 41, 501–504. [CrossRef]

26. Drissi, H.; Refait, P.; Genin, J.M.R. The oxidation of Fe(OH)2 in the presence of carbonate ions: Structure ofcarbonate green rust one. Hyperfine Interact. 1994, 90, 395–400. [CrossRef]

27. Refait, P.; Abdelmoula, M.; Genin, J.M.R. Mechanisms of formation and structure of green rust one inaqueous corrosion of iron in the presence of chloride ions. Corros. Sci. 1998, 40, 1547–1560. [CrossRef]

28. Swietlik, J.; Raczyk-Stanislawiak, U.; Piszora, P.; Nawrocki, J. Corrosion in drinking water pipes:The importance of green rusts. Water Res. 2012, 46, 1–10. [CrossRef] [PubMed]

29. Larson, T.E.; Skold, R.V. Laboratory studies relating mineral quality of water to corrosion of steel and castiron. J. Am. Water Works Assoc. 1958, 14, 285–588. [CrossRef]

30. Liu, H.Z.; Schonberger, K.D.; Peng, C.Y.; Ferguson, J.F.; Desormeaux, E.; Meyerhofer, P.; Luckenbach, H.;Korshin, G.V. Effects of blending of desalinated and conventionally treated surface water on iron corrosionand its release from corroding surfaces and pre-existing scales. Water Res. 2013, 47, 3817–3826. [CrossRef][PubMed]

31. Li, M.J.; Liu, Z.W.; Chen, Y.C.; Wu, Y.Y. Effect mechanism of flow velocity on iron release from pipe surfacesin drinking water distribution systems. In Proceedings of the 37th IAHR World Congress, Kuala Lumpur,Malaysia, 13–18 August 2017.

32. Ali, M.A.; Dzombak, D.A. Effects of simple organic acids on sorption of Cu2+ and Ca2+ on goethite.Geochim. Cosmochim. Acta 1996, 60, 291–304. [CrossRef]

33. Weng, L.P.; Koopal, L.K.; Hiemstra, T.; Meeussen, J.C.L.; Van Riemsdijk, W.H. Interactions of calcium andfulvic acid at the goethite-water interface. Geochim. Cosmochim. Acta 2005, 69, 325–339. [CrossRef]

34. Boxall, J.B.; Dewis, N. Identification of discolouration risk through simplified modelling. In Proceedings ofthe World Water and Environmental Resources Congress, Anchorage, AK, USA, 15–19 May 2005; pp. 1–10.

35. Boxall, J.B.; Saul, A.J. Modeling discoloration in potable water distribution systems. J. Environ. Eng. 2005,131, 716–725. [CrossRef]

36. Boxall, J.B.; Prince, R.A. Modelling discolouration in a Melbourne (Australia) potable water distributionsystem. J. Water Supply Res. Technol. Aqua 2006, 55, 207–219. [CrossRef]

37. Vreeburg, J.H.G.; Boxall, J.B. Discolouration in potable water distribution systems: A review. Water Res. 2007,41, 519–529. [CrossRef] [PubMed]

38. Husband, S.; Boxall, J.B. Discolouration risk management for trunk mains. Water Distrib. Syst. Anal. 2010,2010, 535–542.

39. Sharpe, R.L.; Smith, C.J.; Boxall, J.B.; Biggs, C.A. Pilot scale laboratory investigations into the impact of steadystate conditioning flow on potable water discolouration. Water Distrib. Syst. Anal. 2010, 2010, 494–506.

40. Fabbricino, M.; Korshin, G.V. Changes of the corrosion potential of iron in stagnation and flow conditionsand their relationship with metal release. Water Res. 2014, 62, 136–146. [CrossRef] [PubMed]

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open accessarticle distributed under the terms and conditions of the Creative Commons Attribution(CC BY) license (http://creativecommons.org/licenses/by/4.0/).